Viewfinder
When taking a picture it is necessary to know not only in what direction to aim the camera, but also how much of the subject will be included in the picture. For this reason, practically every camera available today has a viewfinder of some kind.
The viewfinder of a single-lens reflex (SLR) camera is relatively complex. Typically, an objective or front taking lens forms an image of the subject (by focusing rays of light reflected from the subject) on a ground glass viewing screen in the viewfinder via a mirror. The mirror is normally positioned at a 45.degree. angle between the front taking lens and a rear film plane to reflect the light rays from the taking lens to the ground glass screen, instead of allowing the light rays to reach the film plane. The image formed on the ground glass screen is right-side-up, but is reversed left-to-right. A roof pentaprism is placed just above the ground glass screen in the viewfinder to provide eye-level viewing of the image and to reverse the image to orient it correctly, right-to-left. When you look through a rear eye lens in the viewfinder, the image you see is substantially the same as will be exposed on a film frame at the film plane. Since the distance from the front taking lens to the ground glass screen is the same as the distance from the taking lens to the film plane, the image exposed on the film frame will be sharp when the image has been similarly focused on the screen. During exposure, the mirror is pivoted upward to the ground glass screen from its 45.degree. position to permit the light rays from the front taking lens to reach the film plane.
Electrostatic Light Valve
An electrostatic light valve (ELV) consists of a transparent flat stationary electrode, a movable coil-biased resilient sheet electrode, and an insulating layer separating the two stationary and coilable electrodes. The coilable electrode has one end that is fixed with respect to the stationary electrode and a free end that is innermost within the coil when the coilable electrode is coiled. The coilable electrode is caused to unroll (uncurl) along the stationary electrode to overly the stationary electrode when an electric potential (voltage) is applied between the two electrodes. As long as the electric potential is applied, the coilable electrode remains unrolled along the stationary electrode. Conversely, when the electric potential is discontinued the coilable electrode is reversed along the stationary electrode to re-coil (re-curl). The coilable electrode can thus act as an electrostatically actuated light valve or light shutter. This is disclosed in prior art U.S. Pat. Nos. 4,248,501 issued Feb. 3, 1981, 4,704,707 issued Nov. 3, 1987, 5,233,459 issued Aug. 3, 1993 and 5,784,189 issued Jul. 21, 1998. The prior art patents are incorporated in this application.